In another post I wrote how to create with festival, lame and sox an mp3 to practice mental addition. That mp3 used festival’s default english voice. What I did to have a spanish version is the following:

This script uses the festival text to speech system, SoX and lame to generate an mp3 to practice mental addition. It basically says two numbers and after a delay says the sum of those two numbers, the delay is supposed to give you time to calculate the sum mentally: